A water soluble polysaccharide from Carthamus tinctorius flowers was found to react with antisera specific to Types III and Types VIII Streptococcus pneumoniae. Introperitoneal inoculation of mice with this polysaccharide caused the generation of splenic antibody-forming cells, similar in specificity to those detected after immunization with Type III pneumococcal polysaccharide (SSS-III). Antiserum reactive to SSS-III was prepared by immunizing rabbits with C. tinctorius polysaccharide. Chemical analysis of the polysaccharide showed it to consist of xylose, fructose, galactose, glucose, arabinose, rhamnose and uronic acid. The analysis of the types of uronic acids present was facilitated by the development of a new chromatographic procedure, capable of differentiating between glucuronic and galactronic acids.